1. Field of the Invention
The present invention relates to an apparatus for generating x-rays, and in particular to an x-ray radiator having an elongated cathode.
2. Description of the Prior Art
U.S. Pat. No. 4,340,816 discloses a radiation source having an anode which is either elongated or arcuately curved, arranged opposite a plurality of cathodes. These cathodes can be individually driven in succession for the emission of electrons. The electrons can then be accelerated onto the anode as an electron beam for generating a ray bundle. The ray bundle that is generated is thereby conically fashioned. The individual, successively generated, conical ray bundles penetrate an exposure subject and are incident on a radiation receiver that is synchronously driven in a direction opposite the drive of the cathodes. A grating can be provided between the anode and each of the individual cathodes, the emission of electrons of each individual cathode being capable of being controlled with its associated grating.
U.S. Pat. No. 4,490,835 discloses an x-ray examination apparatus having an x-ray tube which generates an x-ray beam which is gated to form a thin rectangular ray fan by a focus-proximate primary radiation diaphragm. This ray fan penetrates an exposure subject and subsequently penetrates a further slot-shaped gating apparatus before it is incident on an image layer carrier. The primary radiation diaphragm and the gating mechanism are aligned relative to one another such that they are adjustable uniformly and in a fixed relationship relative to one another above the image layer carrier for preparing an x-ray exposure of a subject. This x-ray examination apparatus allows x-ray exposures to be produced that have a low proportion of scattered rays. This is desirable since the scattered radiation contains no information about the exposure subject and deteriorates the x-ray exposure.
A large part of the useful x-ray cone of the x-ray tube is blanked by the focus-proximate primary radiation diaphragm, so that only a slight part contributes to the generated x-ray for imaging. The x-ray tube is thus highly stressed in order to provide the x-ray dose needed for producing an x-ray exposure.
British Patent No. 949 312 discloses a cathode of an x-ray tube for generating a uniform and elongated electron emission on the anode. To this end, this cathode comprises an elongated, uncoiled glow wire that is convexly arcuately shaped opposite the propagation direction of the electrons. A metallic shielding having a slot that accepts the glow wire is provided, whereby the front surfaces thereof, which project beyond the glow wire in the direction of the anode, are also convexly arcuately shaped. Glow wire cathodes have a high evaporation rate of the material that emits the electrons during the operation of the x-ray tube, as a result of which the service life is limited. Moreover, the electron emission of an elongated (uncoiled) glow wire is relatively low.
U.S. Pat. No. 3,833,494 discloses a cathode for an electrical discharge tube that has a high electron emission and a long service life. This cathode is composed of a rhenium carrier on which a lanthanum hexaboride layer is applied and sintered in a cataphoretically. A pronounced formation of boride occurs, however, during the operation of the cathode which can lead to the rapid exhaustion and, thus, to the rupture of the rhenium carrier. The service life of this known cathode is consequently reduced.
U.S. Pat. No. 4,752,713 discloses a glow cathode for an electron tube having high emission capability. This glow cathode is composed of a heat-resistant, metallic or ceramic member serving as carrier and a metallic activation substance that promotes the electron emission. This activation substance is composed of an alloy of a group VIII metal and rhenium and an element from the group of Ba, Ca, La, Y, Gd, Ce, Th, U, or by an intermetallic compound of the same elements. This activation substance covers the entire surface of the carrier and may be, for example, a lanthanum and platinum alloy.